In recent years, versatile memory cards (IC cards) each including a nonvolatile memory device such as a flash-memory, have been widely used in the market. Further, there have been developed such a functional module card (IC card) having the same interface as that of a memory card, and including a memory device and other functions (for example, a radio communication function). On the other hand, terrestrial digital broadcasting is being started. For example, if a function of receiving terrestrial digital broadcasting is installed into a functional module card, and a host apparatus can attach the functional module card thereto, the host apparatus is very attractive to users.
A conventional card-type electronic tuner is disclosed in the Japanese patent laid-open publication No. 5-14133. In the conventional card-type electronic tuner, the whole electronic tuner circuit thereof is contained in a card-type case. The card-type electronic tuner is connected to a host apparatus via a connector of the host apparatus, and the card-type electronic tuner transmits a received video signal and a received audio signal to the host apparatus via a video signal output terminal and an audio signal output terminal thereof, respectively.
Generally speaking, a versatile IC card (for example, a functional module card such as an SD card or a memory stick) communicates with a host apparatus by use of a master-slave communication (in which the host apparatus generates a communication clock) in which the host apparatus operates as a master apparatus and the functional module card operates as a slave apparatus. In the master-slave communication, the host apparatus operating as the master apparatus determines a frequency of a transmission clock and a timing of data transmission. In addition, the versatile IC card does not have any communication pathways (for example, an exclusive line for transmitting video signals or audio signals) for communicating with the host apparatus, other than a pathway for communicating with the host apparatus by use of the master-slave communication.
On the other hand, an IC card having the function of receiving terrestrial digital broadcasting reproduces a clock generated by a broadcasting station from received data, processes the received data using the clock, and stores processed data (a broadcasting signal) in a built-in buffer RAM thereof. The host apparatus reads out the broadcasting signal received by the IC card, and then outputs the broadcasting signal using its own transmission clock.
As described above, the clock generated by the broadcasting station and a clock generated by the host apparatus are independent of each other. On the other hand, the digital broadcasting signal is continuous relative to a time axis. Accordingly, when there is even a slight difference between a clock according to which the broadcasting station outputs the broadcasting signal and a clock according to which the host apparatus processes and outputs the broadcasting signal, the difference accumulates in process of time, and this leads to such a problem that a continuity of the broadcasting signal outputted from the host apparatus cannot be maintained.
Concretely speaking, conventionally, when an attempt to provide the versatile IC card including the function of receiving terrestrial digital broadcasting is made, the following problem occurs. When the versatile IC card including the function of receiving terrestrial digital broadcasting is attached to the host apparatus, and the host apparatus reads out the broadcasting signal received by the IC card and outputs the broadcasting signal, the following problem arises. In case where a frequency of the clock according to which the broadcasting station outputs the broadcasting signal is slightly higher than a frequency of the clock according to which the host apparatus outputs the broadcasting signal (a video signal and an audio signal, or a video signal or an audio signal (this kind of combination is simply referred to as a video signal and/or an audio signal hereinafter)), data received by the IC card but not read out by the host apparatus accumulates. Then, if a volume of accumulated data exceeds the capacity of the buffer RAM, the accumulated data overflows from the buffer RAM. As a result, part of the broadcasting signal is discarded without being outputted from the host apparatus. In this case, skipping of the video signal and/or audio signal outputted from the host apparatus occurs.
In case where the frequency of the clock according to which the broadcasting station outputs the broadcasting signal is slightly lower than the frequency of the clock according to which the host apparatus outputs the broadcasting signal (the video signal and/or audio signal), data to be outputted from the host apparatus is exhausted eventually. In this case, a time period without any video signal and/or audio signal to be outputted from the host apparatus is generated (for example, blackout or freezing in the video signal and a silent period in the audio signal). In both cases described above, in a display and/or a speaker for outputting the video signal and/or audio signal outputted from the host apparatus, a problem of causing discontinuity, such as skipping of the video signal and/or audio signal and a period without any video signal and/or audio signal, occurs.
The present invention is directed to solve the above-mentioned problem, and an object of the present invention is therefore to provide an electronic apparatus and a method of controlling the same, for reading a digital data stream including a video signal and/or an audio signal outputted from an external apparatus according to a clock different from a clock for the data stream, and for outputting the video signal and/or audio signal substantially in synchronization with the clock for the data stream (for outputting the video signal and/or audio signal without causing any discontinuity relative to the time axis).
Another object of the present invention is to provide an electronic apparatus (a host apparatus for an IC card), capable of attaching thereto such an IC card having the function of receiving the digital broadcasting signal and being connected to the electronic apparatus via a versatile interface, capable of reading out the broadcasting signal received by the IC card, and capable of outputting the broadcasting signal without causing any discontinuity relative the time axis. In addition, a further object of the present invention is to provide a method of controlling the electronic apparatus.